Porous membranes are often employed as semi-permeable barriers between two or more miscible fluids. In these applications, the membranes control the transmission of components between the fluids, and in the absence of overriding intermolecular forces based on charge, magnetism, dipoles, etc., they can generally be thought of as acting like sieves. As such, fluid components smaller than pores of the membrane can travel from one membrane surface to the other, but substances larger than the pores are prevented from doing likewise. This function is exemplified by the use of membranes as filters to remove particles from liquids or gases.
If a membrane contains a range of pore sizes, the largest pore determines the largest and smallest fluid components that will pass through or be retained by the membrane, respectively. Membranes with maximum pore sizes smaller than about 0.02 .mu.m are commonly referred to as ultrafine, while those with maximum pore sizes between about 0.02 .mu.m and about 10 .mu.m (more typically about 1 .mu.m) are considered microporous. Such membranes are often used by the electronics and pharmaceutical industries to remove particulate impurities from fluids (i.e., liquids and gases), and for reasons of economics and convenience it is preferred that these filtrations be performed rapidly and reliably. Membrane permeability and strength are therefore properties that are almost as important as pore size.
It is difficult to produce strong, permeable, micro and ultrafine membranes, and it is particularly difficult to create such membranes from polytetrafluoroethylene (PTFE). This material has many attractive qualities for use in membrane applications, such as superior chemical inertness and high mechanical stability at a range of temperatures. Strong, highly porous PTFE membranes were first produced by about the 1970's, and their preparation is described in U.S. Pat. No. 3,953,566 (Gore) and U.S. Pat. No. 4,187,390 (Gore). The method described in these patents comprises two fundamental steps: (a) the rapid stretching of unsintered PTFE extrudates to create pores, and (b) heat treatment of the stretched material to increase the mechanical strength thereof.
Since these patents, two general strategies have dominated attempts to prepare strong, highly porous PTFE membranes with smaller pore sizes. One common strategy has been to adopt the basic method of the '566 patent, but to modify and optimize the individual steps thereof. For example, U.S. Pat. No. 5,476,589 (Bacino) discloses an improved protocol of transverse and longitudinal stretching of unsintered PTFE to provide thin PTFE membranes with maximum pore sizes reportedly as small as 0.125 .mu.m.
The sintering level of the stretched PTFE also has been subject to variation. Maximum pore sizes allegedly as low as 0.3 .mu.m are obtained when semisintered PTFE is stretched according to the method of U.S. Pat. No. 5,234,739 (Tanaru et al.), while maximum pore diameters allegedly as low as 0.052 .mu.m are produced if two or more fused sheets of fully sintered PTFE are stretched as described in U.S. Pat. No. 5,510,176 (Nakamura et al.). However, the stretching of fully sintered PTFE is difficult to perform, as the resulting membrane can be fragile and easily damaged. Two advantages of the present inventive method are that porous PTFE membranes of any sintering level can be produced, and the inventive membranes possess excellent mechanical strength.
A second common strategy for preparing a porous PTFE membrane has been to use a pore-forming filler, such as NaCl, instead of using a stretching protocol such as described in the '566 patent. In these methods, the filler is mixed with particles of PTFE, the mixture is transformed into a thin film, and the filler is removed from the film (e.g., by washing with hot water, acids, etc. to dissolve the filler) to create pores. This procedure was used to prepare membranes with maximum pore sizes of at least 0.1 .mu.m in U.S. Pat. No. 4,863,604 (Lo et al.).
The aforementioned procedures all require the execution of complex sequences, and the quality of the resulting membranes can vary considerably if the individual steps (e.g., heating, fusing, stretching, etc.) are not carefully performed. Moreover, the porous PTFE membranes prepared by these methods have maximum pore sizes larger than 0.05 .mu.m.
There exists a need for microporous and ultrafine PTFE membranes of relatively high mechanical strength, permeability, and chemical inertness, and for methods of producing those membranes that is relatively simple and reproducible. The present invention provides such membranes and such methods. These and other advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.